Factors affecting the binding of clay fillers in paper produced from high pulp yields

The properties of paper can be improved greatly by incorporating clay fillers into the paper sheet or applying a clay coating on the paper surface. Filled paper has considerable advantages over unfilled paper including better optical and printing properties. Fundamental studies on aqueous systems containing fibres, fillers and retention aids have been carried out in order to develop a better understanding of the way in which clay fillers are incorporated into the paper sheet. This study has concentrated on the binding of clay to high yield pulp fibres produced from Eucalyptus species and Pinus radiata using cationic retention aids. Scanning electron microscopy shows that the kaolin particles tend to coat the pulp fibres rather than forming aggregates. The surface coverage of kaolin on the fibres can be correlated with the pH of the pulp suspension and is also dependant on ionic strength. Differences in surface area of pulps caused by fibrillation also affect the binding of clay fillers to high yield pulps. FTIR microscopy and FTIR-ATR spectroscopy have been used to look at different fibre types and their admixtures with kaolin. The FTIR studies have shown the presence of kaolin in filled paper from its characteristic peaks. The binding of the retention aid on model surfaces for high yield pulps has also been studied by in situ FUR- ATR spectroscopy. The surface charge of the pulps is important in determining the conditions when deposition of filler particles on the pulp fibres occurs. The surface charge as a function of pH and ionic strength of both bleached and unbleached mechanical and chemical pulps produced from Eucalyptus species and P. radiata have been determined using potentiometric titration. The effects of the retention of lignin and hetnicellulose during high yield pulping were evaluated by comparison of the surface properties of the high yield pulps with those of bleached and unbleached eucalypt and P. radiata lcraft pulps. High yield pulps are commonly bleached by oxidative lignin-retaining bleaching methods. The effect of these bleaching procedures on the surface chemistry of these pulps was studied by surface titration, determination of total acid group content and ultraviolet-visible and FTIR spectroscopy. The high yield eucalypt pulps have higher surface charges and acid group contents than the high yield P. radiata pulps. The surface charge and acid group contents of both high yield pulp types increase on peroxide bleaching. The unbleached eucalypt and P. radiata high yield pulps have considerably higher surface charges and higher acid group contents than the corresponding 'craft pulps. The surface charges of the high yield pulps increase on peroxide bleaching while the surface charges of the kraft pulps decrease on chlorine/chlorine dioxide bleaching. The differences in behaviour between the different pulp types can be explained by the different lignin and hemicellulose contents and differences in the structure of the lignin and hemicellulose components. Oxidative reactions on peroxide bleaching lead to the increased acid group content and increased surface charge. The adsorption of cationic retention aids on the pulps can be correlated with their surface charge.